Actuators for printers must have the capability of consistent operation at high speeds and high repetition rates at high impact force levels for long periods. The problem with known solenoid devices which limits achieving optimum operating results is the high mass of the armature or moving structure. A further problem is that the stator structure using coil and magnetic elements have not been able to efficiently provide the amount of energy to obtain the required velocity and impact force. Attempts to increase magnetic efficiency have usually produced structures which are increasingly complex and have an increased mass thereby reducing the force to mass ratio.
U.S. Pat. No. 3,838,370, issued Sept. 24, 1974 to T. Ueno et al discloses a solenoid magnet having an annular coil between two annular magnetizable stator poles mounted inside a magnetic casing. An armature assembly has two annular magnetizable bodies with the same spacing as the stator poles so as to be simultaneously receivable within the central openings of the coil and stator poles. The armature bodies are connected internally by a rod which uses magnetic material for providing a magnetic flux path through the armature core bodies and the connecting rod.
U.K. Patent Application GB2004504A of Exxon Research and Engineering Co. published Apr. 4, 1979 discloses a print hammer comprising a stator consisting of a winding within a stationary magnetic structure comprising end pole pieces and a cylindrical casing which generates a flux for imparting an impact force to a solid cylindrical magnetic core connected to a non-magnetic impact member.
U.S Pat. No. 4,306,206, issued Dec. 15, 1981 to J. L. Meyers discloses a solenoid device where a cylindrical coil is located between stator poles mounted within a magnetic casing. A cylindrical armature has a magnetic central core and a magnetic peripheral core ring. The core ring and the core define a flux carrying path between a pair of axially spaced cylindrical armature pole surfaces. The armature further has a pair of radially polarized axially spaced annular permanent magnets adjacent the armature pole surfaces.